In general, a non-reciprocal circuit device such as an isolator or a circulator has a function to pass a signal only in a transmitting direction, while blocking the transmission of a signal in an opposite direction. Such isolator and circulator have a distribution-constant type and a lumped-constant type. FIG. 16 shows the internal structure of a distribution-constant-type, non-reciprocal circuit device, and FIG. 17 shows its cross section. This non-reciprocal circuit device comprises a microstrip line member 35 having three lines radially extending from a circular center portion, which may be called a central conductor, a pair of garnet plates 30, 30 disposed on both sides of the circular center portion of the microstrip line member 35, and permanent magnets 20, 20 disposed on both sides of the garnet plates 30, 30 for applying a DC magnetic field thereto, between upper and lower metal cases 10a, 10b. The number of permanent magnets 20 may be one. The lines of the microstrip line member 35 are connected to terminals 150a-150c of connectors 120a-120c arranged on side surfaces of the metal case 10b. 
FIG. 18 shows the appearance of a distribution-constant-type, non-reciprocal circuit device described in JP 2003-124711 A, and FIG. 19 shows its internal structure. This non-reciprocal circuit device 1 comprises an upper iron plate 13, a permanent magnet 20, a lower iron plate 15, a ground plate 16, two garnet ferrite plates 30, a microstrip line member 35 sandwiched by two ferrite plates 30, and a ground plate 16, in this order from above between a metal case 10 and an upper lid 12. The microstrip line member 35 is usually formed by a copper plate as thin as 0.1-0.25 mm, and comprises a resonance portion (substantially triangular center portion) resonating in a TM110 mode, three lines 35a-35c radially extending from the resonance portion, impedance converters each as long as λ/4 and disposed in each line 35a-35c for impedance matching, and input/output electrodes 36a-36c each disposed at a tip end of each line 35a-35c. The input/output electrodes 36a-36c project from the metal case 10 to be soldered to a circuit board.
When current is supplied to the microstrip line member 35, a high-frequency magnetic field is generated from the garnet plate 30. Because the permanent magnet 20 generates a rotating magnetic field in the garnet plate 30, the polarization plane of the high-frequency magnetic field input to any one of the lines 35a-35c rotates, giving an output only to a predetermined line.
Investigation has been conducted so far to miniaturize distribution-constant-type, non-reciprocal circuit devices, but their miniaturization has been difficult because the size of garnet plates is determined by the operating frequencies of the non-reciprocal circuit devices. Proposals also have been made to increase the performance of permanent magnets, and to use one permanent magnet, despite their limits.
In addition, the positional deviation of constituent parts lowers the electric characteristics of non-reciprocal circuit devices. A DC magnetic field applied from the permanent magnet 20 to the garnet plate 30 should be uniform, but the positional deviation of parts such as the garnet plate 30, the microstrip line member 35, the ground plate 40, the resin member 60, etc. provides a non-uniform magnetic field, making the impedance of input/output terminals 36a-36c different from a designed level, and thus failing to achieve the desired electric characteristics. The positional deviation of parts may occur not only in assembling, but also by impact during use, etc. Thus proposed is the bonding of input/output terminals 36a-36c to peripheral sides of the garnet plate 30. However, the bonding needs pluralities of steps, an adhesive may spread to a main surface of the garnet plate 30, and an adhesive may form parasitic capacitance between the input/output terminals and the ground plate 40.